Pensieri di un lunatico minore

Class extensions rock

Earlier, I wrote about extending classes, I wanted to give a clear example of what I personally think is a good use for this. I’m working on some time-travel stuff. As I’m writing some tests to talk about what I want to the code I’m working on to do, I get to this:

TimeTravelTest>>testToday
    | stock version |
    stock := Stock new.
    version := StockVersion price: 12.25.
    stock versionAt: (Date readFromString: '1/12/2006') put: version.
    self assert: (stock versionAt: (Date readFromString: '1/12/2006')) price = 12.25.

That works just fine, but it doesn’t really read very well. So, I created a class extension, and added a new message asDate to CharacterArray that just looks like this:

CharacterArray>>asDate
    ^Date readFromString: self.

so that I can do this instead:

TimeTravelTest>>testToday
    | stock version |
    stock := Stock new.
    version := StockVersion price: 12.25.
    stock versionAt: '1/12/2006' asDate put: version.
    self assert: (stock versionAt: '1/12/2006' asDate) price = 12.25.

That, to my mind, reads much better, and is a clear example of how extending a class can be a “good thing,” if used correctly. It actually creates a situation where the flow of the code is clearer and more explicit about what you are trying to do. I haven’t actually introduced any new dependency between the two classes, but simply made one more expressive.

I would love to use the Chronos Smalltalk library, but as of yet it is not capable of parsing strings, as near as I can determine, even though it can do everything else, including dealing with leap-seconds. Converting from the VisualWorks Date class to a Chronos class seems silly at this point.